Filament-protecting device and shock resistant lamp

ABSTRACT

Incandescent lamps connected in circuits to keep a maintenance current flowing through the lamp filament when the lamp is off to improve the ductile-brittle properties of the filament. Lamps are also disclosed which have an internal impedance element to be used in the circuits for producing the maintenance current.

United States Patent Inventors Ronald C. Koo

Weehawken; Joel Shurgan, Washington Twp., both of NJ. Appl. No. 868,506 Filed Oct. 22, 1969 Patented Nov. 9, 1971 Assignee Duro-Test Corporation Bergen, NJ.

FlLAMENT-PROTECTING DEVICE AND SHOCK RESISTANT LAMP 13 Claims, 7 Drawing Figs.

U.S. Cl 315/94, 315/71 Int. Cl l'l0lj 7/44, HOSb 39/00 Field of Search 315/49, 50, 51, 71, 94

[56] References Cited UNITED STATES PATENTS 1,749,520 3/1930 Voorhoeve 3l5/7l X 2,081,801 5/1937 Dunkel 315/7l 3,148,305 9/1964 Pearson 315/71 X Primary Examiner- Roy Lake Assistant Examiner- Lawrence J. Dahl Allorney- Darby & Darby ABSTRACT: Incandescent lamps connected in circuits to keep a maintenance current flowing through the lump filament when the lamp is off to improve the ductile-brittle properties of the filament. Lamps are also disclosed which have an internal impedance element to be used in the circuits for producing the maintenance current.

PATENTEBHnv s :97:

mm m 0% T 8 m W u ATTORNEYS FILAMENT-PROTECTING DEVICE AND SHOCK RESISTANT LAMP The mechanical properties of the tungsten filament used in an incandescent lamp play a major role in determining lamp life. This is particularly true for lamps used for street service, for example, traffic and sign lamps, in which the filaments are subjected to high stresses induced by mechanical shocks as well as by acoustical vibration. The filament has to withstand these stresses not only when it is in operation and is hot, but also when it is cold. This means that the filament should preferably be designed so that it is sag resistant at elevated temperatures and also cold-shock resistant, that is, resistant to fracture while the filament is not in operation and is cold. It has been found that it is the poor cold-shock resistance of tungsten filaments used in street service lamps which is primarily responsible for the premature failure of this type of lamp.

In the past several approaches have been tried to minimize premature failure of street service lamps brought about by the high stresses induced by mechanical shocks and acoustical vibrations. These approaches include the design of ruggedized filament supports for street service of a different construction from that found in lamps which are not subjected to such stresses. Also, the filament leads of street service lamps have been designed to dampen vibration. Another approach used is to improve the ductility of the tungsten filament material by alloying it with another material. One such prior art tungsten filament has alloyed with it 3 percent rhenium.

All of the various approaches discussed above, and others, reduce to some extent the premature failure of the filaments of street service lamps, but do not entirely eliminate it. One reason for this is that the ductile-brittle transition temperature of recrystallized tungsten is in the range of 150 C.300 C. The ductile-brittle transition temperature is defined as the temperature above which the material is highly ductile and below which it becomes completely brittle. The ductile-brittle transition temperature is characterized by a sudden loss of ductility with a decrease in temperature, and is typical of most body-centered cubic metals such as tungsten, chromium, and iron. This ductile-brittle behavior is different from the behavior of common face-centered cubic metals such as copper and aluminum which still exhibit ductility at temperatures as low as 4.2 K.

The present invention relates to circuits for use with incandescent lamps and to incandescent lamps themselves which provide a protective arrangement to improve the ductile-brittle transition temperature behavior of incandescent lamps, and more particularly of incandescent street service lamps. In accordance with the invention, incandescent lamps and circuits therefore are disclosed in which a heating or maintenance current is provided to the lamp filament during the time when the lamp is turned off. The maintenance current is selected to be of a magnitude which is sufficient to heat the tungsten filament to a temperature in the range from substantially near to above the ducile-brittle transition temperature. This serves to reduce or eliminate brittle fracture and coldshock fracture of the tungsten filaments due to shocks and vibrations by keeping the filament operating near or in the ductile temperature range at all times.

It is therefore an object of the present invention to provide an incandescent lamp which is operated with a predetermined amount of current even when in the off condition, to improve the ductile-brittle temperature behavior characteristics.

Another object is to provide a traffic signal lamp having an internally mounted element for keeping the lamp on with a predetermined amount of heating current for the filament during the time when the lamp is otherwise inoperative.

An additional object is to provide an incandescent lamp with a tungsten filament and an operating circuit therefor which maintains the operating temperature of the filament near or above its ductile-brittle temperature at all times.

Other objects and advantages of the present invention will become more apparent upon reference to the following specification and annexed drawings in which:

FIG. 1 is a schematic diagram illustrating the operating principles of the present invention;

FIG. 2 is a schematic diagram of a circuit used with flashertype lamps;

FIG. 3 is a schematic diagram of a circuit used for operating three lamps to maintain a current through the two lamps of the circuit which are on at any given instant;

FIG. 4 is a schematic diagram of a two-lamp circuit with an arrangement for supplying the idling current to the lamp which is off;

FIG. 5 is a view of a T-type lamp made in accordance with the present invention;

FIG. 6 is an elevational view of another type of lamp made in accordance with the present invention; and

FIG. 7 is a schematic diagram of another circuit according to the invention.

FIG. 1 is a circuit diagram illustrating the basic features of the invention. Here, a lamp 10 is shown which has the usual tungsten filament 11. The tungsten filament can be of the type previously described which is alloyed with a small amount of residual metal, such as rhenium, to improve its shock resistance. It can be of any suitable type, such as coiled or coiled-coil. An impedance element 13, illustratively shown in the form of a resistor, is connected in series with the lamp and a suitable AC or DC power supply source designated by the terminals 15 and 16. A switch 18, shown as a single pole, single throw switch, is connected across the impedance element 13.

In operation of the circuit of FIG. 1, the filament '11 of lamp 10 is brought to full brilliance by closing the switch 18. This shorts out the impedance element 13 so that maximum current will flow through the filament 11. With the switch 18 open, the lamp is ofi, but there is still a circuit to the power supply 15 and 16 through the impedance element 13. The value of the impedance element is selected to be such that a sufficient amount of maintenance current will be passed through the filament 11 to maintain it at a temperature which is somewhat below, just at or somewhat above the ductile-brittle temperature of the tungsten material. I

FIG. 2 is a diagram of another lamp circuit according to the present invention in which two lamps 20 and 22 are used having respective tungsten filaments 21 and 23. One terminal of each of the two lamps are connected together and returned to the power supply terminal 16. The other (left) terminal of lamp 20 is connected through a switch 25 to the other power supply terminal 15 through a switch 27. An impedance element 13 is connected across the two left-hand terminals of the lamps 20 and 22. r

In operation of the circuit of FIG. 2, when the switch 27 is closed, with the switch 25 is open, the lamp 22 is directly across the power supply terminals 15,16 and in a parallel circuit with the series connection of the impedance elements 13 and the lamp 20. The lamp 22 will operate at full light output and there will be a lower value maintenance current flowing through the lamp 20 which will be sufficient to keep the tungsten filament 21 in lamp 20 near or above the ductile-brittle transition temperature but below the temperature of incandescence. Flashing of lamp 20 is accomplished by opening switch 25 and closing switch 27 simultaneously. This places the lamp 22 directly across the power supply terminals 15,16 and the series-connected impedance element 13 and lamp 22 across the terminals 15,16. The major portion of the current will flow through the lamp 20 which now becomes incandescent. At this time, a maintenance current flows through the impedance element 13 and the lamp 22, thus maintaining the temperature of the filament 23 in lamp 22 at the desired ductile-brittle temperature range.

The circuit shown in FIG. 2 is suitable for sign lamps of the so-called walk-don't walk flasher types. In these signs, the two lamps are alternated on and off. Also, one of the lamps can be flashed for a predetermined time at a desired rate. This can be accomplished by any suitable arrangement, such as driving the switches by a motor.

FIG. 3 shows a circuit arrangement applied to a fixture having more than two lamps, such as traffic signal signs having three lamps (red, green and amber). Here, three lamps 37, 39 and 41 are shown having the respective filaments 38, 40 and 42. The right-hand sides of each of the lamps 37, 39 and 41 are connected together and in turn connected to terminal 16 of the power supply. The left-hand terminals of each of the three lamps are connected through a respective network formed by an impedance element and a parallel-connected switch to the other power supply terminal 15. Lamp 37 has the series-connected element 44 which is shunted by a switch 46; lamp 39 has the series-connected impedance element 48 and shunting switch 50; and lamp 41 has the series-connected impedance element 52 and shunting switch 54.

Closing any one of the switches 46, 50 or 54 connected to a respective lamp 37, 39 or 41 and leaving the other two switches open connects that lamp across the power supply terminals 15, 16 and causes maximum current to flow through the lamp having its switch closed. A lower current is maintained through the other two lamps, whose respective switches are left open, of a value determined by the value of the respective impedance 44, 48 or 52. For example, closing switch 46 and opening switches 50 and 54 connects lamp 37 across terminals 15,16, making it have maximum current and incandescence. At the same time, each of the lamps 39 and 41 have minimum heating currents since their respective impedances 48 and 52 are connected in the supply line to terminal 15. Similarly, to turn on the lamp 39 and to keep lamps 37 and 41 off, but with the maintenance current flowing, requires closing of switch 50 and opening of switches 46 and 54. To turn on lamp 41 requires closing of switch 54 and opening of switches 46 and 50. By suitable operating the three switches, such as by a programmed or a motor-driven switching arrangement, flashing cycles in any combination of lamps can be obtained.

The circuits described above with respect to FIGS. 1 through 3 show the impedance elements located external to the lamp. However, it is quite possible, and in many cases quite desirable, to build the impedance element directly into the lamp. FIG. 4 shows such an arrangement wherein two lamps 60-1 and 60-2 are provided having the respective tungsten filaments 62-1 and 62-2. Each of the lamps also has an internally mounted impedance elements which, in this case, is shown as a respective filament coil 64-1 and 64-2. The filament coils 64-1 and 64-2 are wound to have a desired resistance to establish the required heating current when the respective lamp is off. While the lamps 60 bear some resemblance to a standard three-way lamp, they are different in that the respective coils 64-1 and 64-2 have a high resistance and are designed not to be heated up to incandescence.

The right-hand terminals of the two lamps 60 of FIG. 4 are connected together and to the terminal 16 of the power supply. The left-hand terminals of the two lamps 60 are also connected together by a lead 66. One terminal ofa switch 65- 1 is connected between the junction of the filament 62-1 and the impedance element 64-1 of the lamp 60-1. The other terminal of switch 65-1 is connected to terminal of the power supply. Similarly, a switch 65-2 has one terminal connected to the junction of the filament 62-2 and coil 64-2 of lamp 60-2 with the other terminal of the switch being connected to terminal 15 of the power supply.

During the operation of the circuit of FIG. 4, when one of the switches 65 is closed, the lamp connected to that switch will receive maximum current and will light. For example, consider that switch 65-1 is closed. This completes the circuit for lamp 60-1 from terminal 15 of the power supply through switch 65-1 and the filament 62-1 back to power supply terminal 16. Thus the filament 62-1 is connected across the power supply and receives maximum current. For lamp 60-2, the current is from terminal 15, through switch 65-1, re-

sistance coil 64-1 of lamp -1, lead 66 and the coil 64-2 and filament 64-2 of lamp 60-2 back to terminal 16 of the power supply. Thus, lamp 60-2 receives a lower value maintenance current since the high-value series-connected impedance elements 64-1 and 64-2 are in the circuit. The operation of the circuit is similar when lamp switch is open and switch 65- 2 is closed. In this instance 60-2 receives maximum current and lamp 60-1 the heating current. In both lamps, the resistance of the coils 64, is selected so that the desired current flow through the filament of the respective lamp when it is off to keep the filament of that lamp at the desired temperature.

FIG. 5 shows a lamp 70 made in accordance with the invention. Here, the tungsten filament 72 is shown connected in series with an impedance element in the form of a resistor or coil of wire 74 within the lamp envelope 71. The bottom end of the impedance element 75 is connected to one of the terminals of a conventional three-way lamp base 77. Nickel wires 78 and 80, respectively, connect the junction between the filament and the impedance element and the upper end of the filament to respective terminals (not shown) on the lamp base 77. The lamp 70 of FIG. 5 can be used in any of the circuits of FIGS. 1, 3 or 4 or any other similar circuit.

The lamps 110 and 112 are connected in series and operate from an AC source. The current is limited by a capacitor 114 which does not consume power, as does the resistor of FIG. 1. A switch 116 shunts the impedance element, capacitor 114. When the switch contacts are closed after the capacitor is charged to its maximum voltage to prevent excessive current from flowing through the contacts, a small current-limiting resistance 115 is used.

Test measurements were made by passing a measured current through two series-connected lamps (type A-19, 60-watt, 60-volt) from a variable voltage transformer, and reading the voltage across a lamp. FIG. 7 shows the test circuit which is similar to the circuit of FIG. 1.

To demonstrate the efficacy of the invention from another point of view, tests were conducted to show the amount by which the in-rush current was reduced by keeping the tungsten filament warm in the off condition of the lamp by using the maintenance current. The in-rush current is defined as that current which initially flows through a lamp with a cold (room temperature) filament. Reducing the in-rush current will reduce the overheating of localized defects, thus prolonging lamp life.

FIG. 6 shows another type of lamp according to the invention, including the usual envelope with stem press 92 mounted on a three-way socket 94. Leads 95 and 96 come out of the stem press to hold the tungsten filament 97 therebetween. These two leads are connected to two terminals (not shown) on the three-way base 94. An impedance element in the form of a resistor 99 or coiled filament is shown having a lead 100 which is connected to another terminal on the threeway socket. The lamp is connected in either of the circuits of FIGS. 1,3 or 4 shown above.

The capacitor-resistor circuit shown in FIG. 7 can be used in place of the resistance elements of any of the circuits of FIGS. 1-3. An added advantage is that the resistor capacitor combination across the switch would reduce arcing on its contacts and thereby increase switch life.

Table I below shows the test results obtained with the circuit of FIG. 7. The test results given are with switch 116 open and the maintenance current flowing. The tests were conducted with various values of resistors and capacitors 114, to vary the maintenance current and thereby the temperature at which the tungsten filament is maintained when the lamp is off. The standard measured against was the amount by which the in-rush current of the lamp was reduced from a circuit in which there was no maintenance current, and the filament allowed to cool to room temperature when off.

In Table l, the following definitions apply:

(1) Voltage across one lamp This is the voltage measured across a lamp 110 or 112 when it is off, i.e. switch 116 open.

(2) Circuit current in ma. This is the current measured in the circuit with switch 116 open.

(3) Lamp resistance in ohms Obtained by dividing l) (4) Wattage of off lamp Obtained by squaring (2) and multiplying by (3 (5) Value of resistor in circuit sistor 115.

(6) Value of capacitor in circuit capacitor 114.

(7) Approximate C. Temperature in degrees centigrade estimated from Langmuirs tables for tungsten wires.

(3) ln-rush current percent This is the reduction in inrush current achieved by having the tungsten filament warm in the off condition due to the maintenance current.

As should be noted from Table I, by providing a maintenance current to keep the temperature above the brittle point of the tungsten filament, the percentage of in-rush current is reduced substantially. Thus, for example, when a sufficient maintenance current is provided to keep the filament at a temperature of approximately 452 C., the in-rush current is reduced to only 30.2 percent of that flowing without the use of the maintenance current. As shown, as the values of the impedance elements are selected to reduce the temperature at which the filament is maintained in the off condition, the percentage of in-rush current increases. For example, when the temperature is below the ductile-brittle point, for example at 202 C., 56.4 percent of the normal in-rush current will flow when the lamp is turned on by closing switch 116. While this is not as efficient as maintaining the lamp at or above the duetile-brittle temperature, nevertheless the in-rush current can be reduced to a value which will reduce overheating of localized defects in the filament, thus prolonging lamp life.

What is claimed is:

1. In a circuit for operation from a power supply source comprising an incandescent lamp having a filament therein, means for electrically connecting said lamp filament to said power supply source, said connecting means including an impedance element and switching means connected to said impedance element, said impedance element and said lamp filament, said switching means being operative in one position to connect said impedance element between said power supply and said lamp filament to provide a current to heat and maintain the lamp filament at a temperature which is below incandescence but which is near or above the ductile-brittle temperature of the filament material and in another position to remove said impedance element from the circuit to provide an operating current to the lamp filament to heat it to incandescence.

2. A circuit as in claim 1, wherein the lamp filament is of tungsten and the heating current is ofa magnitude sufficient to keep the temperature of the filament in the range from about 250 C. to 450 C.

3. The circuit of claim 1, wherein said impedance element is a resistor.

4. The circuit of claim 1, wherein said impedance element is a capacitor.

Measured value of re- Measured value of 5. The circuit of claim 1 further comprising a second incandescent lamp of the same general type as the first-named lamp, said second lamp also having a filament, and a second switching means, said two switching means connected to said power supply, to the filaments of the two lamps and to said impedance element to selectively connect said impedance elements to the filament of one ofsaid lamps to provide the heating and maintenance current thereto and to the filament of the other of said lamps to provide the operating current thereto.

6. The circuit of claim 1 further comprising second and third incandescent lamps of the same general type as the firstnamed lamp, each ofsaid second and third lamps having a filament, at respective terminal for each end of each lamp filament, said connecting means electrically connecting one terminal of each of the three lamps together, a switching means and an impedance element for each lamp, said impedance element connected in series with the other terminal of a respective lamp, and a respective switching means having its contacts connected in parallel with a respective impedance element for shorting out the respectively connected impedance element, each said impedance element when connected in series with its respective lamp supplying current to the respective lamp filament to provide a current thereto to heat and maintain the filament at a temperature below incandescence but which is near or above the ductile-brittle temperature of the filament material,

7. The switching circuit of claim 1 further comprising a second incandescent lamp of the same general type as the first-named lamp, said second lamp having a filament, a terminal for each end of a respective lamp filament, said connecting means including means for electrically connecting one terminal of the filaments of each of said two lamps together, a respective impedance element for each lamp electrically connected in series with the filament of a respective lamp, means for electrically connecting the free end of the impedance elements, a switching means for each of said lamps, one terminal of each switching means connected to the junction of the im pedance means and filament of a respective lamp, and means electrically connecting the other terminal of each said switching means, each said impedance element when connected in series with its respective lamp supplying current to the respective lamp filament to provide a current thereto to heat and maintain the filament at a temperature below incandescence but which is near or above the ductile-brittle temperature of the filament material.

8. The circuit of claim 1, wherein said lamp comprises an envelope, a filament mounted within said envelope, an impedance element electrically connected in series with said filament, first and second lead wires respectively connected to the free ends of said filament and impedance elements, a third lead wire connected to the junction of said filament and impedance element, a base on which said envelope is mounted, and means connecting said three lead wires to respective ter minals on said base.

9. An incandescent lamp for operating from a source ofcurrent comprising an envelope, a filament mounted within said envelope, an impedance element electrically connected in series with said filament, first and second lead wires respectively connected to the free ends of said filament and impedance elements, a third lead wire connected to the junction of said filament and impedance element, a base on which said envelope is mounted, and means connecting said three lead wires to 11. An incandescent lamp as in claim 10, wherein said impedance element is a resistance wire.

12. An incandescent lamp as in claim ll. further comprising means for mounting said filament and said impedance element in a straight line configuration generally parallel to the axis of the envelope.

13. An incandescent lamp as in claim 11, further comprising means for mounting said filament and said impedance element comprising a stern within said envelope with said three leads being mounted on said stem. 

1. In a circuit for operation from a power supply source comprising an incandescent lamp having a filament therein, means for electrically connecting said lamp filament to said power supply source, said connecting means including an impedance element and switching means connected to said impedance element, said impedance element and said lamp filament, said switching means being operative in one position to connect said impedance element between said power supply and said lamp filament to provide a current to heat and maintain the lamp filAment at a temperature which is below incandescence but which is near or above the ductile-brittle temperature of the filament material and in another position to remove said impedance element from the circuit to provide an operating current to the lamp filament to heat it to incandescence.
 2. A circuit as in claim 1, wherein the lamp filament is of tungsten and the heating current is of a magnitude sufficient to keep the temperature of the filament in the range from about 250* C. to 450* C.
 3. The circuit of claim 1, wherein said impedance element is a resistor.
 4. The circuit of claim 1, wherein said impedance element is a capacitor.
 5. The circuit of claim 1 further comprising a second incandescent lamp of the same general type as the first-named lamp, said second lamp also having a filament, and a second switching means, said two switching means connected to said power supply, to the filaments of the two lamps and to said impedance element to selectively connect said impedance element to the filament of one of said lamps to provide the heating and maintenance current thereto and to the filament of the other of said lamps to provide the operating current thereto.
 6. The circuit of claim 1 further comprising second and third incandescent lamps of the same general type as the first-named lamp, each of said second and third lamps having a filament, a respective terminal for each end of each lamp filament, said connecting means electrically connecting one terminal of each of the three lamps together, a switching means and an impedance element for each lamp, said impedance element connected in series with the other terminal of a respective lamp, and a respective switching means having its contacts connected in parallel with a respective impedance element for shorting out the respectively connected impedance element, each said impedance element when connected in series with its respective lamp supplying current to the respective lamp filament to provide a current thereto to heat and maintain the filament at a temperature below incandescence but which is near or above the ductile-brittle temperature of the filament material.
 7. The switching circuit of claim 1 further comprising a second incandescent lamp of the same general type as the first-named lamp, said second lamp having a filament, a terminal for each end of a respective lamp filament, said connecting means including means for electrically connecting one terminal of the filaments of each of said two lamps together, a respective impedance element for each lamp electrically connected in series with the filament of a respective lamp, means for electrically connecting the free end of the impedance elements, a switching means for each of said lamps, one terminal of each switching means connected to the junction of the impedance means and filament of a respective lamp, and means electrically connecting the other terminal of each said switching means, each said impedance element when connected in series with its respective lamp supplying current to the respective lamp filament to provide a current thereto to heat and maintain the filament at a temperature below incandescence but which is near or above the ductile-brittle temperature of the filament material.
 8. The circuit of claim 1, wherein said lamp comprises an envelope, a filament mounted within said envelope, an impedance element electrically connected in series with said filament, first and second lead wires respectively connected to the free ends of said filament and impedance elements, a third lead wire connected to the junction of said filament and impedance element, a base on which said envelope is mounted, and means connecting said three lead wires to respective terminals on said base.
 9. An incandescent lamp for operating from a source of current comprising an envelope, a filament mounted within said envelope, an impedance element electrically connected in series with said filament, first and second lead wires respecTively connected to the free ends of said filament and impedance elements, a third lead wire connected to the junction of said filament and impedance element, a base on which said envelope is mounted, and means connecting said three lead wires to respective terminals on said base, said impedance elements when operating in series with said filament and receiving current from said power supply providing a current to said filament to heat and maintain it at a temperature which is below incandescence but which is near or above the ductile-brittle temperature of the filament material.
 10. An incandescent lamp as in claim 9, wherein said filament is of tungsten material and the filament is maintained at a temperature in the range of from about 250* C. to about 450* C.
 11. An incandescent lamp as in claim 10, wherein said impedance element is a resistance wire.
 12. An incandescent lamp as in claim 11, further comprising means for mounting said filament and said impedance element in a straight line configuration generally parallel to the axis of the envelope.
 13. An incandescent lamp as in claim 11, further comprising means for mounting said filament and said impedance element comprising a stem within said envelope with said three leads being mounted on said stem. 